[clang] [clang] Use llvm::count (NFC) (PR #143228)

[Kazu Hirata via cfe-commits]

kazutakahirata wrote:

> You should always have a summary, for this one it could have simply been "replace std::count with llvm::count in Sema::CheckBaseSpecifier"

@shafik Ack.

https://github.com/llvm/llvm-project/pull/143228